1. Field of the Invention
The present invention relates to a surface acoustic wave filter in which the propagation direction of a surface acoustic wave can be varied by use of a reflection edge, and especially which has excellent reflection efficacy and a very small chip size.
2. Description of the Related Art
Conventionally, surface acoustic wave devices have been used as IF filters for mobile communication equipment. Such surface acoustic wave filters include double mode surface acoustic wave devices operable in a double mode using longitudinally coupled or transversely coupled are used. In such surface acoustic wave devices, the pass-band width of the filter is significantly affected by the electro-mechanical coupling coefficient of a material for a piezoelectric substrate used in a surface acoustic wave filter. Therefore, the pass-band width of the filter can not be significantly changed by arrangement and configuration of electrodes such as interdigital electrodes (IDTs), reflectors, or other elements.
In recent years, ring pass filters, Z pass filters, or other filters have been attractive because the filter pass-band widths can be varied by arrangement and configuration of the IDTs, reflectors, or other elements.
FIG. 5 is a plan view of a ring pass filter 101.
As shown in FIG. 5, first and second IDTs 103 and 104, first reflectors 103a and 103b, and second reflectors 104a and 104b are disposed on a piezoelectric substrate 102.
One of the interdigital electrodes of the first IDT 103 is connected to an input terminal 105, and the other interdigital electrode is grounded. One of the interdigital electrodes of the second IDT 104 is connected to an output terminal 106, and the other interdigital electrode is grounded.
The first reflectors 103a and 103b are arranged to sandwich the IDT 103 and are arranged along a line with the IDT 103. The reflectors 103a and 103b are arranged to have inclined shapes to reflect a surface acoustic wave excited by the first IDT perpendicularly to the input direction to the respective reflectors.
The second reflectors 104a and 104b are arranged to sandwich the second IDT 104 along a straight line with each other, in the surface acoustic wave propagation direction. Moreover, the reflectors 104a and 104b are arranged to have inclined shapes to reflect a surface acoustic wave reflected by the first reflectors perpendicularly to the input direction to the respective reflectors, that is, toward the second IDT 104.
In the conventional ring pass filter or Z pass filter having such a configuration as described above, Rayleigh waves are used as a surface acoustic wave to be propagated.
In the case of Rayleigh waves, a large number of the electrode fingers constituting a reflector are required. The reflection coefficient of the above-mentioned reflector is considerably varied depending on the respective factors such as materials for the substrate, cut angles, propagation directions, materials for the electrodes, electrode thicknesses, wavelengths for the electrode fingers, widths of the electrode fingers, and other factors. Thus, it is very difficult to make an optimum design for a surface acoustic wave filter such as a ring pass filter, a z pass filter, or other filter. It takes a great amount of time to achieve the optimum design.
Furthermore, when a reflector for use with a Rayleigh wave, having the above-described structure is used, the reflection efficiency is low. Thus, there arises a problem that only surface acoustic wave devices having a low energy efficiency can be produced.
Moreover, in a filter having a conventional structure, the reflector has a predetermined frequency characteristic. Accordingly, in order to match the frequency characteristic with that of the IDT, it is required that the number of the reflectors should be a predetermined value or higher. For example, at least 80 reflectors are required for 100 MHz to 300 MHz. Accordingly, in the filter having the conventional structure, the required number of reflectors is large, and simultaneously, the lengths of the reflectors become larger. Thus, since there are time delays between surface acoustic waves reflecting from the respective electrode fingers constituting the reflectors, problematically, deviations between the group delay time characteristics become large.
In particular, as shown in FIG. 6, the group delay time characteristic (D) in the pass-band of the frequency characteristic (C) of the filter tends to be increased at both of the ends thereof. The deviation at the center frequency xc2x13 MHz is about 100 nS.
Moreover, the larger the above-described deviation of the group delay time characteristic (D) becomes, the more the phase of a signal changes. Thus, there arises the problem that moderate response to an input signal can not be obtained. Accordingly, in a communication device containing such a surface acoustic wave filter, for example, in a communication device such as a digital system portable telephone, there is the danger that a voice is interrupted, or no voice is output during call, though a radio wave is received.
Moreover, the surface acoustic wave filter experiences the problem that since the length of the reflector itself becomes large, the overall length of the filter is increased. This is an obstacle to miniaturization of the surface acoustic wave filter.
In order to solve the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter in which deviation of the group delay time characteristic is minimized, and miniaturization can be easily achieved.
According to a preferred embodiment of the present invention, a surface acoustic wave filter includes a piezoelectric substrate and first and second IDT provided thereon. The first IDT is disposed on the surface of the piezoelectric substrate. The second IDT is disposed on the surface of the piezoelectric substrate and is not arranged along a straight line with the first IDT in the surface acoustic wave propagation direction. A first reflection edge is provided on the piezoelectric substrate, and positioned along a straight line with the first IDT in the surface acoustic wave propagation direction so that a Shear Horizontal (xe2x80x9cSH typexe2x80x9d) type surface acoustic wave excited by the first IDT is input to the first reflection edge and is reflected therefrom at an angle relative to the input direction. A second reflection edge is provided on the piezoelectric substrate and arranged along a straight line with the second IDT in the surface acoustic wave propagation direction, so that the SH type surface acoustic wave reflected from the first reflection edge is input to the second reflection edge and is reflected therefrom at an angle relative to the input direction, toward the second IDT.
The distance between the center of the first reflection edge and the center of the second reflection edge of the piezoelectric substrate is preferably substantially equal to approximately xcex/2xc3x97n (n is an integer) xc2x1xcex/8, in which xcex represents the wavelength of a surface acoustic wave propagating the piezoelectric substrate.
The distance between the center of the outermost electrode finger in the first IDT and the center of the first reflection edge, or the distance between the center of the outermost electrode finger in the second IDT and the center of the second reflection edge is preferably substantially equal to approximately xcex/2xc3x97m (m is an integer) xc2x1xcex/8, in which xcex represents the wavelength of a surface acoustic wave propagating the piezoelectric substrate.
The surface acoustic wave filter of preferred embodiments of the present invention can be incorporated in a duplexer, and a communication device using such a duplexer.
According to preferred embodiments of the present invention, no reflector having a plurality of electrodes is required. As a result, it is not necessary to determine the many parameters required for design of the reflector. Thus, the filter design process for preferred embodiments of the present invention is very easy, efficient and not time consuming.
According to preferred embodiments of the present invention, a surface acoustic wave filter such as a ring-pass filter, a Z pass filter, or other filter is configured so that a surface acoustic wave is reflected from reflection edges. Therefore, there is much less difference between the delay times of surface acoustic waves propagated from a reflection edge to the IDT on the output side. Thus, the group delay time characteristic becomes flat, and the deviation can be reduced.
Moreover, the reflection edges have a higher reflection efficiency as compared with a reflector including electrode fingers. Thus, the loss caused by the reflection elements is eliminated, and moreover, the propagation loss is very low, since the propagation distance of an SH wave is very short. Thereby, the insertion loss can be improved. Moreover, as the reflection parts, the reflection edges are employed instead of the reflectors. Thereby, the filter can be easily reduced in size.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the present invention is not limited to the precise arrangements and instrumentalities shown.